An Anti-lock Brake System (ABS) is often included as standard equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. A typical ABS includes a plurality of solenoid valves mounted within a control valve body and connected to the vehicle hydraulic brake system. Usually, a separate hydraulic source, such as a motor driven pump, is included in the ABS for reapplying hydraulic pressure to the controlled wheel brakes during an ABS braking cycle. The pump is typically included within the control valve body while the pump motor is mounted upon the exterior of the control valve body.
An ABS further includes an electronic control module, which has a microprocessor. The control module is electrically coupled to the pump motor, a plurality of solenoid coils associated with the solenoid valves and wheel speed sensors for monitoring the speed of the controlled wheels. The control module is typically mounted upon the control valve body to form a compact unit, which is often referred to as an ABS electro-hydraulic control unit.
During vehicle operation, the microprocessor in the ABS control module continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is operative to actuate the pump motor and selectively operate the solenoid valves in the control unit to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled wheel brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver.
Referring now to FIG. 1, there is shown a sectional view of a typical ABS solenoid valve 10 mounted upon an ABS control valve body 11. The valve 10 includes an axially shiftable armature 12 which is biased in an upward direction by a spring 13 such that a ball valve, generally represented by a reference numeral 14, is maintained in a normally open position. The ball valve 14 cooperates with a valve seat member 15 which is mounted in the valve body 11. The armature 12 is slidably disposed within a valve sleeve 16 having a closed end. A solenoid coil 20 is carried by the valve sleeve 16 and surrounds the armature 12. The coil 20 is enclosed by a metal flux return casing 21. An annular flux ring 22 is disposed within the open end of the flux casing 21. The flux return casing 21 and flux ring 22 complete a magnetic flux path which passes through the armature 12 and the valve seat member 15.
The solenoid coil 20 is of conventional design, comprising a winding 23 formed from multiple turns of an insulated magnet wire having a round cross-section, such as #28xc2xd magnet wire. The coil wire is helically wound upon a plastic bobbin 24. A pair of terminal pin supports 25 extend in an axial direction from the top of the bobbin 24. Each of the supports 25 is molded over a terminal pin 26. A lead wire 27 of the coil winding wire is wound around the base of each of the terminal pins 26 and soldered thereto. The pins 26 are electrically coupled to the ABS microprocessor.
When it is necessary to actuate the valve 10 during an anti-lock braking cycle, an electric current is supplied through the terminal pins 26 to the coil 20. The current establishes a magnetic field in the armature 12, which pulls the armature 12 in a downward direction, closing the ball valve 14. When the current is interrupted, the magnetic field collapses, allowing the spring 13 to return the armature 12 to its original position, thereby reopening the ball valve 14. An ABS control unit also typically includes other solenoid valves, such as normally closed solenoid valves (not shown), which have structures similar to the normally open valve 10 described above.
The invention is directed to a coil assembly comprising a bobbin for supporting a coil. The bobbin supports a metal ferrule. A metal bracket is releasably coupled to the bobbin. The bracket holds the ferrule in place. The ferrule and the bracket are arranged to cooperatively serve as a return path for magnetic flux. The invention is also directed to a method for assembling a coil assembly.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.